Manufacture of aluminum parts of complex shape by die casting is a well known industrial process. In this process, molten aluminum alloy is injected under high pressure into a metal mold, where it is allowed to solidify and then removed from the mold as a final product.
A major factor in the expense of aluminum die casting is the cost of the dies, including die inserts. Especially where the part to be made has a complex shape, the cost of die preparation can be very high. Therefore, it is important that a die have a reasonable useful life, since replacing the die too often can make the overall die casting process uneconomic.
Many factors contribute to the gradual wearing out of dies used for aluminum die casting over time. Repeated expansion and contraction of the die from thermal cycling leads to formation of cracks in the die surfaces, known as “heat checking.” Molten aluminum also tends to alloy with the die surfaces causing attack or “washout” of these surfaces over time. Jets of molten aluminum can also cause some die surfaces to soften and abrade over time. Deposits of molten aluminum also adhere or “solder” to other die surfaces causing the shape of the mold cavity to change.
Most dies used for aluminum die casting are made from H13 tool steel hardened to about 44 to 48 HRC. See page 18*32, Metals Handbook, ® 1985 American Society for Metals. This material shows good hardness and resistance to thermal cycle cracking over time. Nonetheless, dies made from this material wear out over time, adding to the expense of the overall die casting process.
Another factor contributing to the overall expense of aluminum die casting relates to the heat conductivity of the mold. Solidification of molten alloy requires that heat be extracted from the mold interior. The faster the heat can be extracted, the faster the molding process can be carried out. In some industrial applications, extraction of heat from the mold is the speed limiting factor of the molding cycle. In these applications, the thermal conductivity of H-13 tool steel constrains faster mold operation, thereby adding to the expense of the overall process.
Accordingly, there is a need for a new material of construction for making dies to be used in aluminum die casting, which material exhibits a superior combination of hardness and thermal conductivity, while also exhibiting the other properties necessary for acceptable mold performance such as resistance against thermal cycle cracking, resistance against washout, resistance against soldering and the like.